Fiber optic communication generally involves modulating optical signals at high bit rates and transmitting the modulated optical signals over optical fibers. For example, in a wavelength division multiplexed (WDM) fiber optic communications system, optical carrier signals at a sequence of distinct wavelengths are separately modulated by information channels and then multiplexed onto a single optical fiber. Efforts continue toward increasing the data capacity of fiber optic communications systems, as well toward increasing the practical transmission distance of fiber optic spans. Although the development of erbium-doped fiber amplifiers (EDFAs) has virtually eliminated fiber attenuation as an obstacle to achieving longer transmission distances, group velocity dispersion and optical fiber nonlinearities continue to represent barriers to increased transmission capability.
Optical fiber nonlinearities begin to manifest themselves as the capabilities of the channel are pushed to their limits through the use of increased signal power, higher bit rates, longer transmission distances, and increased numbers of channels. One physical mechanism associated with at least one fiber nonlinearity is the optical Kerr effect, in which the refractive index of an optical fiber varies in accordance with the intensity of an optical signal. The variation of the refractive index modulates the phase of the optical signal, resulting in adverse effects such as self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). Another physical mechanism associated with at least one fiber nonlinearity is the Raman effect, arising from energy transfers between the propagating photons and the vibrational/rotational modes of the glass molecules in the fiber.
Because of fiber nonlinearities, there may be substantial restrictions on one or more of signal power, the number of WDM channels that can be carried, bit rates per channel, permissible fiber dispersion amounts, and maximum regenerative repeater spacings. It would be desirable to provide an optical fiber communications system in which nonlinearities induced by optical fibers are at least partially compensated, while also providing for the necessary dispersion compensation. It would be further desirable to provide such optical fiber communications system using fiber spans that can be physically realized using known, off-the-shelf optical components. Other issues arise as would be apparent to apparent to one skilled in the art upon reading the present disclosure.